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Old 08-04-2016, 06:32 AM   #1
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Surging Onan 12.5 diesel genset

Hi all,
Curious if anyone has had and had corrected a surging 12.5 Quiet Diesel genset?
Mine is at 2600 hours and has been great, came home from a trip last week and it was hunting just a little, almost hard to even hear it but I do see it in the voltage levels (up and down voltages). I have taken the covers all the way off, replaced the front end bearing, brushes, cleaned the slip rings, new cooling belt, new fuel filter, cleaned AC connections, checked for leaks, oil level, obvious defects and induction air cleaner...ect

Basically nothing found on google about it. Lots of info out there about gas generators surging.

I have the 3 cylinder Isuzu engine.
Any input appreciated.
Thanks
Windecker
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Old 08-05-2016, 08:43 AM   #2
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I was hoping you would get some help here, as we have a 12.5 also but do not have that problem. Just looking for info too.
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Old 08-05-2016, 08:48 AM   #3
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When you changed the fuel filter, was it full ?

If it's pulling air in before the pump, you can get a surge.

If you run it under a heavy load, it should struggle to maintain speed of its a fuel problem.
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Old 08-05-2016, 11:18 PM   #4
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Had the exact problem last year on our 10KW Onan. The Actuator Speed Control had failed. p/n 150-2700. $332.40. Labor $536. Work done at Cummins Rocky Mountain in Avondale (Phoenix) AZ. Their Onan specialist is very good.

Cheers,
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Old 08-16-2016, 03:45 PM   #5
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Surging

Thanks,
The filter change had no affect and loaded or unloaded did not matter either except that it seemed to happen more after a cold start following getting to temperature (meaning it did not surge until it had warmed up a little, loaded or unloaded).
Thanks
Windecker

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Originally Posted by twinboat View Post
When you changed the fuel filter, was it full ?

If it's pulling air in before the pump, you can get a surge.

If you run it under a heavy load, it should struggle to maintain speed of its a fuel problem.
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Old 08-16-2016, 04:00 PM   #6
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Actuator

Thanks for the heads up.
Actually, when I was checking things out I had pulled the electrical connectors off of the Governor actuator (maybe the same thing?) to check the connections and resistance. Seems to me it said it should be between 2.7 and 2.8 ohms. My Fluke showed 3.0 consistently. I have to admit, only .2 ohms off of spec on a simple crudely wired two wire relay looking thing seemed pretty close.
I plugged it back in and it has ran perfectly since. Only have about 6 hours on it now but has been perfect. Loaded and unloaded has been right on.

I am thankful because as most of us know, generators are not easily worked on and most of the time you have to leave your coach somewhere for who knows how long...

Hope that is the last time I see the issue but time will tell. I will remember this advice though, thanks.
Windecker
Update, I just found that 150-2700 actuator in the parts manual that you replaced, yup, that was it. I know where to go if this happens again

Quote:
Originally Posted by bobcovey View Post
Had the exact problem last year on our 10KW Onan. The Actuator Speed Control had failed. p/n 150-2700. $332.40. Labor $536. Work done at Cummins Rocky Mountain in Avondale (Phoenix) AZ. Their Onan specialist is very good.

Cheers,
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Old 02-01-2017, 11:04 PM   #7
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Hey Windecker - any more issues with the generator surging? mine is doing the same thing - just hunting ever so slightly - after it has run awhile and is really warmed up. seems worse with a light load. I changed the fuel filter - and the pump (boy that was fun), checked lines, checked flow - after reading this thread I also checked the connections and resistance at the actuator - all check good... 10.0 HDCAA
Thanks,
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Old 02-03-2017, 05:32 AM   #8
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Happy for me, no, I have not had any further issues. Now to be fair, I only have maybe 40-50 more hours on it since the issue but things have been well.

If it happens to me again I will start by cleaning the connections at the regulator again and if it still continues I would replace it.

I wish I could help further but I am not a diesel mechanic and these generators seem to have black magic in them for whatever reason LOL.

Good luck and let me know if you figure it out, others will benefit.
Will
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Old 02-03-2017, 11:04 PM   #9
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Thanks Will, maybe I'll clean the slip rings, check the brushes etc. It only does it when thoroughly warmed up, and definitely is worse / more frequent under a light load. Don't really want to change the control unit at $800! when I checked the actuator I got the same .2 error you did... but then my old MAC tools meter shows .2 with the leads touching so I figure its right there. I had a guy tell me I need to blow the carbon out of the spark arrestor - and I found some discussions on that topic - but I can't see that causing the problem, and it sounds like a mess, so that will be my last resort - right before the board. probably won't get to it for a while but I'll check back in when I do.
Thanks again,
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Old 02-05-2017, 08:07 AM   #10
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Surging

Kurt,
I am not sure that the slip rings have anything to do with it, I just cleaned mine and replaced the brushes while I was in there. I guess it is possible it could play into it as the genset is searching for a frequency and voltage output which do change with resistance and RPM.
I doubt it is the control module, I would be that relay or governor issue.

Frustrating but not causing any real harm.
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Old 07-30-2017, 09:29 PM   #11
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Will, just an FYI, I did clean the slip rings, replaced the brushes, and found a control board that I could try - no help - replaced the actuator (interestingly the new one ohmed out the same as the old one) and it runs rock steady now.
Thanks,
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Old 07-31-2017, 06:47 AM   #12
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Well that is good news! I guess it is just not a common problem, either that or folks don't care or notice.
Glad you got going for less than $15K (replacement).
Windecker

Quote:
Originally Posted by Kurt Averill View Post
Will, just an FYI, I did clean the slip rings, replaced the brushes, and found a control board that I could try - no help - replaced the actuator (interestingly the new one ohmed out the same as the old one) and it runs rock steady now.
Thanks,
Kurt.
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Old 10-01-2019, 10:09 AM   #13
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Hi Everyone,

I am having a similar problem with my Onan 12,500 Isuzu generator. It surges for a while, but then shuts itself off. It used to stay on a lot longer (hours), but has gradually reduced the amount of time it takes before s shuts itself off. Now it's down to minutes.

I didn't notice anyone saying that theirs was shutting off. Any idea of where I should start?
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Old 10-01-2019, 11:48 AM   #14
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Never mind! We got enough panels off to see a piece of belt lying on the base/floor. Looks like it's not getting coolant circulated. That makes things a little easier (and not that expensive).
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